1. Field of the Invention
The invention relates to oximeters which measure levels of blood oxygenation and, in particular, to a plethysmograph system for pulse oximetry having a reduced noise sensitivity.
2. Description of the Prior Art
Oximeters are photoelectric devices which measure the oxygen saturation of blood. Historically, these devices were first used in clinical laboratories on samples of blood taken from patients. In recent years, non-invasive oximeters have been developed and are now widely used in intensive care units to monitor critically ill patients and in operating rooms to monitor patients under anesthesia. Early non-invasive devices relied on dialization of the vascular bed in, for example, the patient's ear lobe to obtain a pool of arterial blood upon which to perform the saturation measurement. More recently, non-invasive devices known as "pulse oximeters" have been developed which rely on the patient's pulse to produce a changing amount of arterial blood in, for example, the patient's finger or other selected extremity. See Yelderman et al., "Evaluation of Pulse Oximetry", Anesthesiology, 59:349-353 (1983), and Mackenzie, N., "Comparison of a Pulse Oximeter with an Ear Oximeter and an In-Viv Oximeter", J. Clin. Monit., 1:156-160 (1985).
Pulse oximeters measure oxygen saturation by (1) passing light of two more selected wavelengths, e.g., a "red" wavelength and an "IR" wavelength, through the patient's extremity, (2) detecting the time-varying light intensity transmitted through the extremity for each of the wavelengths, and (3) calculating oxygen saturation values for the patient's blood using the: Lambert-Beers transmittance law and the detected transmitted light intensities at the selected wavelengths.
Prior to the present invention, the patient's extremity has been exposed to the selected wavelengths sequentially, that is, a first light source, such as, a red-emitting LED, has been turned on for a period of time and then turned off, and then a second light source, such as, an IR-emitting LED, has been turned on and then off. See, for example, U.S. Pat. No. 4,167,331 and 4,407,290. Alternatively, it has been proposed to pass broadband light through the extremity and separate the transmitted light into two components using appropriate filters. See U.S. Pat. No. 3,998,550.
Each of these approaches lead to complex and/or expensive devices. For example, filters which are able to adequately separate IR from red light are generally expensive. Also, two light sensors, one for each wavelength, are required for the filter approach. Accordingly, with this approach it is difficult to produce an inexpensive, disposable sensor module, as is required for operating room and other uses.
In the case of the sequential exposure approach, the apparatus must keep track of which light source is active. This involves deploying switches throughout the signal processing portion of the apparatus whose states are changed as the different sources become active. In addition, delay or "dead" times must be incorporated in the system to ensure that the measured transmittance relates to just the source which is currently active and not to a combination of the two sources. Moreover, the sources must be switched rapidly and the delay times must be kept short so that within each on-off/on-off cycle, the amount of blood and other characteristics of the patient's extremity remain essentially constant.
In addition to the foregoing, both approaches suffer from interference problems due to ambient light and signals produced by random electrical and optical energy sources. In particular, changing amounts of ambient red and/or IR radiation can lead to errors in the oxygen saturation measurement. Both of these radiations are normally present in, for example, an operating room as a result of general lighting and the presence of active electrical equipment. Variations in the levels of these radiations at the location of the oximetry sensor can result from such simple activities as movement of personnel or equipment within the operating room. Moreover, even constant amounts of these background radiations pose problems for existing oximeters since they saturate the sensor and/or lead to low signal to noise ratios.
Development of an oximeter with a high signal-to-noise ratio is extremely important. Currently, pulse oximeters are not very useful in the treatment of some of the sickest people. Lowering of the pulse decreases the signal-to-noise ratio, and therefore, negatively affects the oximeter reading. Patients that have cardiac problems or that are very cold have weak pulses. The elderly generally have very weak pulses in the extremities. There has also been great difficulty is using a pulse oximeter to measure the oxygen saturation of a fetus. If a pulse oximeter, less susceptible to noise, could be developed, it would be capable of providing a reading even when a patient has a weak pulse, and would thereby make pulse oximetry available to a larger population of patients.
In an attempt to deal with the ambient radiation problem, existing oximeters have incorporated complicated circuitry to compensate for background radiation and have placed the sensors in hoods or other packages designed to minimize the amount of ambient light which can reach the sensor element.
In addition to the ambient radiation problem, existing oximeters are also highly sensitive to signals created by electrical apparatus. Electrical devices in hospitals, such as electro-surgical instruments, can generate radio frequency signals that a plethysmograph system can pick up. It is desirable then to minimize the sensitivity of the system to interfering signals from sources of this nature.
A known technique for minimizing the interfering signals described above is to alternately drive the light sources by a signal having a frequency which is not present in artificial light or characteristic of other medical instrumentation. While effective for rejecting unwanted signals, the energization of the light sources in alteration by the driving signal mandates that the detector be synchronized with the driving signal for correct demodulation. As the following discussion will show, this arrangement requires undesired widening of the receiver bandwidth resulting in an introduction of noise to the system.
A second known technique for eliminating the interfering signals described above is disclosed in U.S. Pat. No. 5,555,882. This technique involves scanning each demultiplexor frequency to determine which has the lowest associated noise. The noise level, associated with the operating frequency, is used to determine the signal-to-noise ratio of the pulse oximeter signals and thereby qualify certain signals from the pulse oximeter. Those pulses associated with a signal-to-noise ratio below a predetermined threshold are rejected and excluded from use in calculating blood oxygen saturation. This technique reduces noise to a certain extent, however, where several interference sources exist, the selection of the optimal modulation frequency can be extremely difficult.
A third known technique for reducing noise in pulse oximeters is disclosed by U.S. Pat. Nos. 4807630 and 4848901. This technique involves the use of dual drive frequencies for the LEDs. This technique has a number of drawbacks. First, implementation of this technique requires the use of a pair of bandpass filters, thereby increasing the cost of the oximeter. Second, use of this technique requires the introduction of a second modulation frequency, which on its own may make the system more receptive to noise. Third, treating the two modulation frequencies differently (giving them different frequencies) may prevent cancellation of error when the red signal and the IR signal are eventually divided by the microprocessor.
A simpler and more effective solution to the interference/noise problem is to adopt an alternate modulation scheme which allows the use of a narrow-band photodetector amplifier. Use of such an amplifier completely avoids the generation of spurious response lobes, as will be discussed in further detail below. Since sensitivity is restricted to a narrow band of frequencies, there is much less opportunity for noise to enter the system. Further, the analysis of harmonics and selection of the optimal modulation frequency is much simplified.
The above mentioned solution may also be useful in improving the design of oximeters incorporating a reflectance sensor which uses back scattered rather than transmitted light. A pulse oximeter incorporating a reflectance sensor has a much smaller amplitude signal to work with. Consequently, a reduction in noise would be extremely useful for such an oximeter.